1. Field of the Invention
The present invention relates generally to an optical medium and a method for manufacturing the optical medium, and more particularly, but not by way of limitation, to an optical disk and a method for manufacturing the optical disk using an improved bonding and centering method for reducing errors caused by conventional bonding processes.
2. Description of the Related Art
Digital Video Disc (DVD) is an optical information recording medium capable of storing a large amount of high definition image data. Due to its various advantages such as good portability and high storage capacity, the DVD has received much attention as the new information recording medium for the next generation.
The diameter and thickness of a conventional DVD are typically 12 cm and 0.6 mm, respectively. Compared to a conventional Compact Disc (CD) having a diameter of 12 cm and a thickness of 1.2 mm, the conventional DVD has exactly one-half of the thickness of the conventional CD. The conventional DVD having the thickness of only 0.6 mm, however, is extremely difficult to carry because it is so thin and susceptible to bending and damages. For this reason, currently available DVDs are formed by bonding two identical disk members to each other, each disk member having a thickness of 0.6 mm. As a result, the currently available DVDs offer good portability because of an increased thickness, and has a high storage capacity since information can be recorded on both sides of the DVD.
To manufacture the above-described DVD, a bonding process is required to bond the two identical disk mediums to each other. The bonding process will be described briefly in conjunction with the entire process of manufacturing the conventional DVD.
First, a photo resist is deposited uniformly on a well-polished glass plate (also known as a glass master substrate), and desired information is recorded on the glass plate by sensitizing the photo resist using a laser beam modulated according to the information to be recorded. A glass master is obtained by developing the photo resist and a metal master is obtained by electroplating the obtained glass master. Then, several mothers can be obtained from the metal master, and subsequently several stampers can be obtained from the mothers. This process is commonly called a mastering process.
A stamper provided by the mastering process is adjusted to fit into an injection molding machine, so that an injection molding process is performed to produce a transparent substrate. The injection molding process injects a polycarbonate resin into the mold containing the stamper, whereby a transparent disk member is obtained.
Next, the obtained transparent disk member is coated with an aluminum reflective film by a sputtering process, which sputters and places metal ions on the transparent substrate.
When the sputtering process is completed, a UV bonding material, e.g., UV resin, is dropped onto the central part of the disk member while the disk is rotated at a high speed. The high-speed rotation of the disk member creates centrifugal force by which the UV bonding material is instantaneously and uniformly distributed over the entire disk member. Lastly, the UV bonding material is cured by irradiating ultraviolet rays on the disk member, whereby a protective layer is formed on the disk member to prevent oxidation of the coated aluminum reflective film.
Then two of the identical disk members formed according to the above-described procedures, each disk member having a 0.6 mm thickness, are bonded to each other to form a conventional DVD. To bond the two disk members, the spin-coating method, hot-melting method, and double-sided tape method are generally used, among which the spin coating method is the most common.
There are two conventional methods, the radical method and cationic method, which use the spin coating process to bond the two disk members to form a conventional DVD. The two methods are similar in that UV bonding materials are applied to the substrate by the spin coating method. A difference between the two methods is that the radical method conducts the bonding process before curing the UV bonding materials, whereas the cationic method completes curing the UV bonding materials before the disk bonding process. The radical method can be considered more common than the cationic method.
FIG. 1 is a flowchart showing a conventional bonding process for forming a conventional DVD. As shown therein, a first disk member is transferred to a bonding machine in Step S101. After loading the first disk member onto a motor shaft in Step S103, a UV bonding material is applied onto the central part of the first disk member while the first disk member is rotated at a low speed in Step S105.
Subsequently, a second disk member is placed on top of the first disk member and the first and second disk members are rotated at a high speed. The centrifugal force created by the high-speed rotation of the first and second disk members uniformly spreads the UV bonding material over the surfaces of the first and second disk members, thereby securely bonding the first and second disk members to each other in Step S107.
In Step S109, ultraviolet rays are irradiated onto the rotating bonded disk members to cure the UV bonding material, and the bonded disk members are unloaded in Step S111, whereby the bonding process is completed and a conventional DVD is manufactured.
The bonding process as shown in FIG. 1, however, has serious problems. For example, the center hole diameter of the finally manufactured DVD exceeds an allowable deviation limit, even though the disk members bonded together satisfy the requirement for the center hole diameter specified by DVD physical format books.
To investigate an error rate caused by the conventional bonding process, experiments of bonding two substrates having a center hole diameter of 15.003 mm have been conducted. The two substrates were made by an injection molding machine by Meiki company and were bonded to each other using a bonding machine with jigs having center hole diameters of 15.001 mm and 15.002 mm. One hundred experiments were performed using each jig and the experimental results indicated that the jig having the center hole diameter of 15.001 mm has an error rate of 22.2% and that the other jig has an error rate of 9.8%.
The centering problem of the conventional bonding process can be solved if the injection molding machine can produce disk members having an identical center hole diameter or if the center hole of the jig can be adjusted to perfectly match that of the substrate. With currently available technology, however, it is almost impossible to satisfy these requirements, such that the centering process of conventional bonding processes has been a major obstacle in the DVD manufacturing process. Moreover, because the bonding process is a significant part of the DVD manufacturing process, any errors caused by the bonding process increase the production cost.
An object of the present invention is to provide an optical medium and a method for manufacturing the optical medium, capable of reducing errors caused by a centering process using a simpler bonding method.
Briefly described, the present invention is directed to a disk medium, including a first disk member having a penetrating hole, and a second disk member having a projecting portion, the projecting portion being placed within the penetrating hole of the first disk member, whereby the first and disk members are combined.
The present invention is further directed to a method of manufacturing an optical medium, including forming a first disk member having a penetrating hole, forming a second disk member having a projecting portion, and positioning the projecting portion of the second disk member within the penetrating hole of the first disk member, whereby the first and second disk members are combined to manufacture an optical medium.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.